1. Field of the Invention
This invention relates to a device for detecting an image. More particularly, the invention relates to a catoptric optical apparatus for forming and detecting an image of an object emitting electromagnetic energy in wavelengths from ultraviolet through infrared.
2. Description of the Related Art
Heretofore, zoom optical devices have used multiple refractive lenses or a combination of refractive lenses and reflective elements to achieve selectively magnified images while correcting for chromatic and monochromatic aberrations. Use of refractive lenses in such devices reduces the amount of energy that reaches a detector element and introduces chromatic aberrations. In addition, each lens results in energy intensity losses because of surface reflections and because of absorption as the energy travels through the lens material. These losses are typically substantial.
The prior art zoom optics generally work very well in energy frequency bands in which the selected lens materials are transmissive, but these zoom optical devices suffer from the disadvantage of being heavy and bulky. In addition, these devices generally require large lens diameters to increase the amount of energy focused on the detector while compensating for reflection and absorption losses.
In the long infrared wavelength spectrum, lens materials, such as flint glass, are not transparent. Thus, to obtain a refractive system sensitive to infrared energy, lenses made of exotic materials, such as silicon, germanium or sodium chloride, must be used. These materials are somewhat transparent at infrared wavelengths, but reflective losses still occur at each lens surface and absorption losses occur as the energy travels through the lens material. Moreover, infrared optical systems are expensive to manufacture, are delicate and may require special protection when they are used in different environments.
Refractive zoom optical systems are used to provide a change in the field of view and correspondingly a change in the magnification of an image. Since the focal point for two different wavelengths of light is different for a given refractive design, careful attention must be paid to the selection of lens materials and to the figure of the various lenses to reduce chromatic aberration to an acceptable level. In addition to chromatic aberration, as the wavelength of the radiation varies, e.g., as the wavelength shortens to ultraviolet and beyond or lengthens to infrared on the other end of the spectrum, energy attenuation as the incoming radiation passes through lens elements becomes important. It is, in general, not possible with currently known lens materials to use a single lens material in ultraviolet, visible and infrared parts of the spectrum.
A reflective device provides two major advantages over refractive designs. First, reflective elements are free of chromatic aberration. This permits all wavelengths from ultraviolet to infrared to be focused at the same point. Secondly, although the reflectivity of mirror coatings may change with wavelength, a mirror is not subject to absorptivity since the electromagnetic energy does not pass through the material.
Wavelength not only affects the optical characteristics of refractive elements but also affects a detector's response. Accordingly, if different portions of the electromagnetic spectrum emanating from a scene being viewed are to be viewed or detected, it is necessary to move detectors having differing wavelength response characteristics into and out of the image formed by the optical system.
It is an object of the present invention to provide an optical system capable of detecting any portion of the electromagnetic spectrum emanating from a scene being viewed.
It is a further object of the present invention to provide such an optical system wherein a desired one of a plurality of detectors can be moved into the image formed by an optical system.
A still further object is to provide a zoom optical device capable of selectively magnifying and detecting the image of an object emitting electromagnetic energy in wavelengths from ultraviolet through infrared without the many disadvantages in cost and construction of the prior art devices.
It is a still further object of the present invention to provide a multispectral band detector with common optics of low cost and complexity wherein absorption losses and chromatic aberration are minimized or eliminated.
Additional objects and advantages of the invention will be set forth in part in the description which follows, and in part will be obvious from the description or may be learned by practice of the invention. The objects and advantages of the invention may be realized and attained by means of the instrumentalities and combinations, particularity pointed out in the appended claims.